The present invention relates to a process for polymerizing ethylene. More particularly, it relates to a process for producing polyethylene having wide molecular weight distribution which has excllent fabricatability such as hollow molding, extrusion molding, which comprises polymerizing ethylene with a novel catalyst having high catalytic activity and long life.
The inventors have studied and found that a catalyst for polymerizing ethylene which has remarkably high catalytic activity and has not disadvantages, can be obtained by using a solid catalytic component obtained by reacting the reaction product (A) with titanium or vanadium halide. (Japanese Patent Publication Nos. 19308/1975, 19309/1975, 154/1976 and 13232/1977.)
It has been found that remarkably high yield of polyethylene per the transition metal halide and remarkably high yield of polyethylene per the solid catalytic component can be attained in the polymerization of ethylene with a catalyst consisting of the solid catalytic component and the organometal compound whereby an adverse effect of the transition metal halide etc. in the polymer is not found even though a complicated and uneconomical step of removing the catalyst is not employed.
This is significant advantages in the practical operation.
In the previously proposed catalytic system, the variation of the catalytic activity during the polymerization is small to maintain high catalytic activity for a long time as the initiation whereby a stable continuous polymerization is attained. The catalyst is highly sensitive to the molecular weight regulator as hydrogen whereby the molecular weight of the polymer can be highly varied by using a small amount of the molecular weight regulator and various grades of the polymers in wide range can be obtained.
The polyethylene obtained by using the previously proposed catalyst system has high bulk density and the powdery form is substantially uniform and spherical whereby the slurry of the polymer and the dried powdery polymer have excellent flowability and the failure in the polymerization such as clogging of the transferring devices is advantageously small.
However, in the previously proposed processes, it is not satisfactorily applied for the production of polyethylene having wide molecular weight distribution.
The narrow molecular weight distribution of polyethylene is preferable in the field of the injection molding etc. whereas it is quite disadvantageous in the fields of the extrusion molding and the hollow molding etc.
When polyethylene having narrow molecular weight distribution is used in the extrusion molding, high extrusion speed can not be obtained to decrease the productivity of the processing machines, and on the other hand, ununiformity of the shape of the molded product and rough surface of shark skin are caused to deteriorate quality of the product.
In the case of the hollow molding, the extrusion speed is also decreased and the shark skin of the surface of the product is caused and strips and pittings are formed to decrease commercial value of the molded product.
The shark skin phenomenon of the surface of the molded product closely related to the molecular weight distribtuion of the polyethylene. The degree of the shark skin is decreased depending upon the increase of the molecular weight distribution.
In General, the flow ratio (FR) is used as a degree of molecular weight distribution and it is a ratio of flowing quantities of a molten polymer under different extruding pressures.
For example, melt indexes are measured at 190.degree. C. under the load of 2.16 Kg and 10.0 Kg by ASTM-D 1238-65T to give MI.sub.2 and MI.sub.10 and FR is given by MI.sub.10 /MI.sub.2.
When MI.sub.2 is similar, larger FR indicates wider molecular weight distribution.
In the hollow molding and the extrusion molding, it is preferable to have FR of higher than 14 in order to obtain a molded product having smooth surface.
In some kind of the previously proposed catalysts, it is possible to increase molecular weight distribution in certain degree by selecting kinds and amounts of the organometallic compound or varying conditions for preparing the solid catalytic component and conditions in the polymerization. However, it is not satisfactory.
On the other hand, various efforts for increasing molecular weight distribution have been made by considering the catalyst especially the solid catalytic component. However, in the previously proposed ones, in general, the catalytic activity is low, and the yield of polyethylene per weight of the transition metal halide or weight of the solid catalytic component, is not high enough. Accordingly, even though a carrier is used for improving the catalytic activity, the advantage for eliminating the catalyst separating step can not be satisfactorily eliminated.